A radar system may be utilized for many purposes in an automotive vehicle. For example, a radar system enables safety features such as collision warning and adaptive cruise control. The effectiveness of such systems may depend upon the performance of the radar system. A radar operates by transmitting an electromagnetic wave that is reflected from an object back to a radar receiver. The reflected signal may be shifted in frequency from the transmitted signal. The radar electronics, using a mixer, may generate a signal that is the difference in frequency between the transmitted and received signals. This frequency difference may then be processed to calculate the range and relative velocity of the object.
A homodyne receiver may down-convert a radar signal directly to a baseband frequency without first converting the signal to any intermediate frequencies. Non-moving (no Doppler content) returns from zero or near-zero range may result in DC or near-DC frequency signals in the baseband. High-level leakage and noise signals and the frequency spread due to oscillator phase noise may mask the DC and low-frequency (near-range) returns.
The noise resulting from the homodyne down-conversion process may be primarily a DC bias noise, sometimes referred to as mixer bias. The bias contributes to noise referred to as 1/f, 1/f2, and 1/f3 noise. This noise, in addition to inherent device noise in the case of 1/fn components, may be the result of equal, or nearly equal, frequency components reacting in the down-conversion mixer and the phase noise of the signals themselves. Leakage of the local oscillator (LO) and radio-frequency (RF) signals may self-mix to DC. The leakage may also reflect off the mixer ports internally due to imperfect matching.
The transmit-to-receive antenna isolation may be greater than 50 dB and radar return signals may be greater than 50 dB below transmit levels. The leakage in the mixer itself may be approximately 20-25 dB below the oscillator signal levels. In automotive radars, the oscillator signal level to the homodyne mixer may only be 3-5 dB below the transmit signal level. Near range target return levels are limited by the R4 roll-off dictated by the radar range equation. In addition, the radar cross section (RCS) of the targets may be physically limited by the illumination of the antenna, which may be a very small spot at near range. Thus, the leakage signals in the mixer may dominate the returns of near range targets with low RCS that have little or no relative motion.
Pedestrians may have a small RCS and low Doppler content. At near range, a return signal from a pedestrian may be masked by the mixer bias noise. In stop-and-go situations, a radar instrumented vehicle may be following a target vehicle to a stop. Issues may arise when the target vehicle exhibits a low RCS (e.g., motorcycles, certain cars). As the vehicle approaches the target vehicle and reduces speed, the target vehicle return may become masked by mixer bias. The result may be reduced performance of functions that rely on the radar system to detect objects.